1. Field of the Invention
The present invention concerns an arrangement for permitting detection of the state of a machining tool rotating in a machining head. This arrangement is especially designed to be used in the field of machine tools and in particular for the detection of the presence and the rotation of a drill mounted on a drilling spindle on a special machine such as those used in the automobile industry.
2. Description of the Prior Art
It is known that in machine tools used for automatic machining, and particularly in multiple-operation machines, it is necessary to maintain a continual watch on the state of the tools and to stop the machine as soon as breakage of a tool has been detected. It is essential that the machine be stopped as quickly as possible since any delay in the detection of such an occurrence results in ruining a large number of parts being machined. Moreover, such tool breakage causes breakage of tools at subsequent stations and damage to the machinery itself. Such is the case, for example, when a tap is damaged because it cannot penetrate where a hole, supposed to have been drilled out at the preceding station, is defective because of breakage of the drill.
To take care of these problems, a known method is to provide special stations after the machining units to verify that the necessary operations have been performed well and that no fragments of broken tools are present in drilled holes, for example. Such special stations commonly have a test plate mounted on the end of a screw jack rod so as to be able to slide along it, the plate being furnished with fingers which can penetrate, under the action of the screw jack, into the properly drilled and cleared holes. In case a bad machining operation occurs at a drilling station, the fingers on the plate cannot penetrate the holes which causes retraction of the plate sliding on the rod of the jack screw which can then actuate a switch stopping the multiple-operation machine.
It is apparent however that the presence of these special stations complicates the operation of the machine. In addition, various types of tool sensors have already been utilized, the action of which can be mechanical, pneumatic or optical, all of which are deficient in some manner.
The arrangements for detection with mechanical sensing devices are cumbersome, of little practical use and in some cases require withdrawal of the tool from its guide tube for checking which risks damaging the tool.
Arrangements with mechanical feelers are also known in which the tool, such as a drill, does not leave its guide tube. The indication given by such detection arrangements are unreliable, however since foreign bodies such as pieces of broken tools, shavings, cutting oil, etc. can act on the feeler in the same way as the tool, the presence of which is to be verified.
Also known are pneumatic detectors consisting of an air jet directed radially near the end of the drill. In such detection arrangements, the indication furnished by the detector can depend on the speed of rotation of the drill, a predetermined value of which can thus be detected. However, a fixed piece like a shaving can completely block the passage of the compressed air across the guide tube and have the same effect on such a pneumatic detector as a drill turning at high speed.